The study of human brain function and its alterations with disease is founded on our increasing understanding of the biochemical nature of the brain. The earliest and most specific changes occurring in diseases of the brain are those that disturb its underlying biochemical processes.
In the past, information about the structure and processes of the human brain were dependent upon biopsies or chemical assays of blood, cerebrospinal fluid, or urine. More recently, development of positron emission tomography (PET) has furthered our understanding of the structure, organization and chemical basis of both normal and diseased cerebral function.
In positron emission tomography, biologically active substrates can be radiolabelled by replacing natural elements of the biochemical constituents of the body with the corresponding radioisotopes. For example, natural isotopes of carbon, nitrogen, and oxygen in biological molecules are replaced with the corresponding radio isotopes carbon-11, nitrogen-13, or oxygen-15. In addition, fluoride-18 can be used in biological molecules. These radioisotopes have short half-lives ranging between 2 minutes for .sup.15 0 and 110 minutes for .sup.18 F, and they all decay by emission of positrons. Positrons, which are positively charged electrons, travel only a few millimeters in living tissue before encountering an electron, resulting in an event that annihilates both particles. The mass of the two particles is converted into two photons traveling in directions approximately 180 degrees from each other with sufficient energy (511,000 electron volts each) to penetrate the bony structures of the head and to be detected externally, for example, by special equipment known as a tomograph.
The tomograph consists of an array of radiation detectors placed circumferentially around the body of the subject. The radiation detectors are connected to a coincidence circuit that records an event from the tissue only if both detectors sense an anihilation photon simultaneously.
The events registered by the detectors are fed into a fast computer which reconstructs images based on the distribution and frequency of radioactive events within the subject. The computer's reconstruction in a section of the imaged object is a quantitative representation of the spatial distribution of the radionuclide used. Detection systems currently employed in PET have a resolution of approximately 5-7 millimeters in the plane of the tomographic section.
Using quantitative tracer methods, PET allows measurement of the changes with time of tissue concentrations of the labelled compound throughout the brain. Therefore a quantitative, non-invasive in vivo technique is provided for tracing such biological processes as hemodynamics, transport phenomena, and neurotransmitter localization via tracer kinetic mathematical models.
Quantitative PET methods have been developed for use in the study of dopaminergic receptor systems, as well as for opiate and benzodiazepine systems. These types of PET studies are transformed into measurements of processes such as receptor affinity and density, neurotransmitter concentration and turnover, ligand off and on rates, and diffusion rates using kinetic and biochemical models.
Studies of the dopaminergic neurotransmitter system have linked it to Huntington's and Parkinsons' diseases as well as to schizophrenia. For instance, the antipsychotic action of neuroleptic drugs has been correlated with blockade of D.sub.2 dopamine receptors. Amphetamines, which elevate synaptic dopamine levels, have been found to induce psychotic states resembling schizophrenia and to exacerbate symptoms of schizophrenic patients.
Increased numbers of D.sub.2 dopamine receptors have been detected in the brains of diagnosed schizophrenic patients in post mortem studies. In some studies these increases were attributed to prior treatment of the patients with neuroleptics, while in other studies increased D.sub.2 sites were found in vivo in drug-free schizophrenic patients. Wong, D. F., et al., "Positron Emission Tomography Reveals Elevated D.sub.2 Dopamine Receptors in Drug-Naive Schizophrenics," Science. 24, pp. 1558-1562. Therefore, interpretation of increased numbers of D2 receptors in brain tissue remains controversial. An in vivo PET study of D.sub.2 dopamine receptor levels comparing two groups of schizophrenic patients--one previously treated with neuroleptics and one group never treated with neuroleptics --has been used to estimate caudate D.sub.2 dopamine receptor densities. Radiolabelled 3-N-[.sup.11 C]methyl-spiperone (half life of .sup.11 C is 20 minutes) was used to quantitate neurotransmitter receptor density and affinity in the brains of living subjects. Other studies have used [.sup.11 C]chlorpromazine, [.sup.11 C]raclopride, [.sup.76 Br]spiperone, [.sup.18 F]spiperone, [.sup.18 F]n-methylspiperone, 3-(2'-[.sup.18 F] fluoroethyl) spiperone, and N-(3-[.sup.18 F]fluoropropyl) spiperone. These studies indicate that a dopamine receptor abnormality, specifically an elevated level of D.sub.2 dopamine receptors, is characteristic of untreated schizophrenic patients. See Arnett, et al., "Improved Delineation of Human Dopamine Receptors Using [.sup.18 F]-N-methylspiroperidol and PET," J. Nucl. Med. 27:1878-82, 1986.
There are several advantages of using .sup.18 F-labeled ligands for PET studies relative to .sup.11 C-labeled compounds. The radiolabelled biologically active substrates used in the D2 receptor studies all have relatively short half-lives, but the half-life of .sup.18 F is 110 minutes, while the half-life of .sup.11 C is only 20 minutes. Therefore, the time between production of the radiolabelled element in the cyclotron and introduction of the biologically active compound containing the radiolabelled element into the subject is about three half-lives for .sup.11 C, but only one half-life for .sup.18 F. Also, the time required after its introduction for the substrate to reach equilibrium in the human body should be no more than one additional half-life. The longer half-life of .sup.18 F provides greater time to reach equilibrium makes .sup.18 F doubly preferred over .sup.11 C for use in PET studies.
.sup.18 F-containing substrates are also preferred over .sup.11 C labelled substrates because of their higher specific activity, or radioactivity per unit mass and lower positron energy. A radiolabelled substrate having high specific activity is especially preferred for D.sub.2 receptor studies because the D.sub.2 receptor sites in the brain are normally in small concentration. As the specific activity of .sup.18 F-labelled substrates is usually five to six times higher than that of .sup.11 C labelled substrates, they are highly preferred for D.sub.2 receptor PET studies.
However, the chemistry of .sup.18 F is known to be difficult. Fluorine gas reacts violently and indiscriminately with organic molecules. The reaction produces a mixture of products that is difficult to separate. Moreover, many reactions involving .sup.18 F are erratic and the yields are low. However, in recent years the knowledge of .sup.18 F chemistry and cyclotron target design has progressed to the point that .sup.18 F-containing radiopharmaceuticals of high purity and high specific activity can be produced.
Although several .sup.18 F radiolabelled dopaminergic compounds have been made with success, to date no dopamine agonists radiolabelled with positron-emitting radionuclides have been shown to retain selectivity of the ligand for dopamine D.sub.2 receptor sites. See Van der Werf, "Synthesis and In Vivo Distribution in Rat Brain of .sup.11 C-Labelled N-Alkylated ADTN Derivatives," Int. S. Radiat. Isot, 35, 8, pp. 377-81, 1984.
In light of these developments, the need exists for new and better radiolabelled biologically active substrates having an affinity for binding to D.sub.2 dopamine receptor sites and for a process of using such substrates to detect the occurrence of abnormalities in D.sub.2 receptor functions. More particularly, the need exists for .sup.18 F-containing dopamine agonists having high specific activity and for the processes of making and using such substrates to determine abnormalities in the dopaminergic system.